Training, information, and encouragement for new amateur radio operators.

Impedance

The standing wave ratio (SWR) is an important topic to hams regardless if they are working HF, VHF, UHF or any other frequency range allocated to radio amateurs. Unfortunately it is technically involved and somewhat complex so is not intuitive or easy for non-technical folks to understand. We’ll give a simplified explanation of SWR here and give you a basic idea of its significance and how hams relate to it.

The simplest way to think of SWR is as a measure of impedance matching. Most commonly it is looking at the impedance differences between transceiver, transmission line (more often coaxial cable), and antenna.

Assuming that all modern radios and cable have 50Ω impedance, the real SWR of interest boils down to the match between the transmission line and antenna.

As we mentioned in a previous post, when impedance of a source equals load impedance the best possible signal coupling occurs. Conversely, when impedances are not the same, signals couple poorly. This is true of all electronics circuitry involving AC waveforms.

In amateur radio SWR focus is on transceiver to antenna coupling where we want to maximize RF power transfer in both transmit and receive modes. When impedances do not match, received signals will be weak or non-existent; when transmitting, power will not radiate well from the antenna. The ideal or “perfect” SWR for best possible transmit/receive coupling is 1:1, meaning a 50Ω radio/transmission line to a 50Ω antenna.

SWR is simply the ratio of two impedances being measured. It is commonly expressed in the X:X format and the larger value is always given first, regardless of which side is higher.

With a 50Ω radio and 50Ω coax cable, a 4:1 SWR could indicate either a 12.5Ω or a 200Ω load (antenna). Similar examples for 50Ω cable are contained in the General class exam pool:

A ham’s main concern with high SWR is significant power reflected back from the load, which stresses the transmitter power amplifier. While a 1:1 SWR is ideal, practically speaking, 1.5:1 or less is good. Many modern transceivers automatically reduce transmit power with a SWR greater than 2:1.

Radio amateurs should be familiar with the term dummy load, which is a RF-friendly substitute for an antenna when testing a transmitter or piece of equipment such as a Watt meter.

A dummy load is somewhat generic, also having industrial and commercial uses. As applied in ham radio, it electrically simulates an antenna to allow a transmitter to be tested without radiating radio waves, typically at 50Ω to match transmitter output impedance.

Dummy loads are rather simple— just a big resistor and some way to dissipate heat, all in a package that must be non-reactive, meaning it provides insignificant capacitance and inductance.

Why must a dummy load be non-inductive? Because of impedance (practically speaking, AC resistance), which increases with frequency based on the formula of inductive reactance XL=2πfL.

Most common power resistors are wire-wound, which have significant inductance. So RF dummy loads must use resistors with little or no inductance.

As an example, this four-resistor series combination using common Dale metal-clad resistors measures 49.4Ω at 0Hz (DC). Sounds like a perfect dummy load, right?

Ever wonder why RF grounds should be flat straps and not regular wires?

This is because ordinary wires are not good conductors at frequencies higher than 50-60Hz. This complicates wiring and bonding requirements.

Impedance (effectively, AC resistance) of a conductor increases with frequency and length due to inductive reactance. The higher the frequency, the greater the impedance.

All conductors have some measurable inductance, and it doesn’t take much to yield significant impedance. At KHz or MHz frequencies, long round wires might present hundreds or even thousands of Ohms impedance; not suitable for grounding.

A good ground has less than one ohm impedance. This is a genuine safety issue.

Since inductive reactance increases with frequency and length, safety grounds and module bonds need to be something other than long round wires when radio frequencies are involved.

When high frequency grounding is required, use short, wide, and flat conductive straps. The high aspect ratio minimizes electrical inductance vs. a round wire, as does a short conductor. This lowers the ground wire’s impedance at higher frequencies.

So now you know. Keep it flat and short (KIFS is a lousy acronym).

It’s not just a suggestion; this one might just bite you if you don’t heed the guideline!

Impedance is an important subject in amateur radio so we want to spend a little time discussing it here. Several topics on this site will involve impedance so it’s good to have this basic concept well understood.

In ham radio work we deal with impedance in transmission lines, antennas, transmitter outputs, receiver inputs, microphones, speakers, headphones, and other devices. Impedance matters everywhere a signal couples to something different.

Basic resistance (R) is what opposes current in a DC circuit, and all components have measurable resistance.

But things get more complicated in AC circuits. Capacitors and inductors (coils) oppose change. This includes alternating current, a characteristic of audio, video and radio frequencies. The properties of capacitance and inductance have well-defined opposition to AC which varies by signal frequency.

All components have measurable capacitance and inductance so there is always some reactance (X) in a circuit. There are two flavors of reactance: capacitive and inductive. Interestingly, they respond oppositely to signal frequency. Inductive reactance (XL) goes up with frequency while capacitive reactance (XC) goes down.

When you add the constant resistance in a circuit to the capacitive and inductive reactance, the result is impedance (Z=R+jX). In broad terms, it can be considered “AC resistance”, which is legitimate when we don’t care about the complex phase angle part of the equation. Resistance plus reactance equals impedance (Z).

Like DC resistance, impedance (AC resistance) is measured in ohms.

OK so far? Click on the many hyperlinks in this article for more detail, along with helpful links below. Don’t worry, you only need to grasp the basics here; high-level math is not necessary for a working knowledge of impedance.

Now that you know what impedance is, the next important thing to understand is that when an AC signal interfaces with a new circuit, the impedances should match.

When impedance of a source (ZS) equals the load impedance (ZL), the best possible signal coupling occurs. Conversely, when impedances are not the same, signals couple poorly.

The maximum power-transfer theorem says that to transfer the maximum amount of power from a source to a load, the load impedance should match the source impedance (ZS=ZL).